1. Field of the Invention
The present invention relates to a variable valve timing system in an engine having two cam-shafts and in particular to a variable valve timing system to vary the valve timing of intake and exhaust valves.
2. Description of the Related Art
A conventional variable valve timing system 100, as shown in FIG. 5, is disclosed in the U.S. Pat. No. 5,067,450. The system 100 is used for an engine (not shown) having two cam-shafts (only one is shown). An apparatus 101 is located on one end of the cam-shaft 102 and comprises outer teeth 103. The outer teeth 103 are geared with outer teeth (not shown) of a pulley (not shown) which is fixed to the other cam-shaft.
A rotating torque of the cam-shaft 102 is transmitted to the other cam-shaft via a case 110, a cylindrical piston 111, a pulley 112 and the not-shown pulley. There are backlash in a geared portion 121 between the case 110 and the piston 111, geared portion 122 between the piston 111 and pulley 112 and geared portion 123 between the pulley 112 and not-shown pulley. In general, a backlash is a cause of a noise since two geared teeth hit each other.
The system 100 comprises an apparatus 130 to decrease the noise. That is, labyrinth groove portion 131 is formed between the case 110 and the pulley 112 and is covered by a cover 132 to fill an oil therearound. Therefore, a movement which is generated when two geared teeth is damped by the aperture 130 and force of the impact is softened.
However, in order to accommodate the apparatus 130, it is necessary to provide the pulley 112 with a large diameter. As a result, a space between two cam-shafts is enlarged and the engine is also enlarged.